The calculation formulas of pulse duration and amplitude of growing current at specified values of maximum velocity and weight of accelerated body by railgun have been obtained. To provide minimum values of railgun current and source strength it is necessary to obtain current impulse close to rectangular shape from the source and to have the most railgun inductance. The dependencies defining power conversion factor of railgun including the energy losses in rails possible in electric arch and remaining energy in the railgun magnetic field are presented for current rectangular impulse. It is shown that conversion factor increases with increasing mass of accelerate body and its maximum velocity as well as with decreasing the railgun length at optimal values of accelerated body's cubic density and its initial velocity.